The use of thumbnails or what we call here Thumbshots is everywhere. Not just on the internet but on your local desktops and laptops.
Apple understands how useful thumbnails can be for their users. The
MAC OS has this Expose feature that displays applications that are open in a thumbnail format. I think there is no other company that understands usability better than these folks here. With their Expose feature, they know their users can navigate their desktops much faster and efficiently than using other means. Our eyes can distinguish small cues so quickly that it can help know which applications that are currently. Microsoft has a similar feature as well called
Windows Flip 3D.
It's no different on the web. Thumbshots or small screenshot previews allow users to pick up colors, structure or format of the website. With this information, the brain can quickly process what the website is about. It could be a familiar site that they have visited or it could be one that looks like an adult site.
Thumbshots can visualize adult sites. More about Thumbshots